Seismic exploration, whether on land or at sea, is a method of detecting geologic structures below the surface of the earth by analyzing seismic energy that has interacted with the geologic structures. A seismic energy source generates a seismic signal that propagates into the earth, where the signal may be partially reflected, refracted, diffracted, and/or otherwise affected by one or more geologic structures such as, for example, interfaces between underground formations having varying acoustic impedances. Seismic imaging systems include one or more sources that can be arranged in various configurations. For example, sources can be placed at or near the earth's surface, on or within bodies of water, or below the earth's surface. Seismic sources can be controlled or uncontrolled. A “controlled source” is a source that deliberately generates seismic signals at the control of the seismic imaging system. A seismic wave that is deliberately generated by a controlled source at the direction of the seismic imaging system is referred to as a “controlled signal” or an “active signal,” and the images resulting from the processing of these signals are referred to as “controlled seismic data” or “active seismic data.” An “uncontrolled source” is a source that produces a seismic wave that is not deliberately generated by the seismic imaging system. A seismic wave that is generated by an uncontrolled source is referred to as an “uncontrolled signal” or a “passive signal.”
Seismic receivers placed at or near the earth's surface, within bodies of water, or below the earth's surface in well-bores are able to detect the seismic signals and record them. The recordings are processed to generate information about the location and physical properties of the subsurface geologic structures that interacted with the seismic signal. A set of recordings taken during a particular time period may be referred to as a “survey.” One or more signals recorded from a single survey can be used to generate an image of the subsurface formations. Such images, referred to as “2D images” or “3D images,” indicate the state of the subsurface formations during the time period in which the survey was taken. Seismic data can also be gathered at difference times. This type of analysis is referred to as “time-lapse” or “4D” imaging. “Permanent Reservoir Monitoring” (PRM) or “Continuous Reservoir Monitoring” (CRM) is used to perform 4D imaging near a reservoir over an extended period of time, though such implementations need not be permanent or continuous. 4D processing of two seismic datasets recorded at different times facilitates the determination of how and where the Earth's properties have changed during that time period. Seismic datasets recorded at different times are referred to as different “vintages.” 3D images from different vintages can be compared to identify differences in the subsurface structures. For example, 3D images from different vintages can be differenced to generate “4D images,” which are also referred to as “4D differences” or “4D effects.” In 4D imaging, the reference dataset can be referred to as the “baseline” survey or vintage, and the dataset measured against the baseline can be referred to as the “monitor” survey or vintage. 3D and 4D images are typically generated from the recording and processing of controlled signals.
Because 4D images are generated from seismic data acquired at different times, 4D images measure changes in subsurface formations over time. For example, 4D images may be developed in a reservoir before and after a period of production. Such 4D images are used to identify reservoir activity of interest such as, for example, fluid movements or changes in fluid or lithological properties in and around a reservoir. Features of a 4D image related to fluid production may be considered “4D signal” or “4D signature” while other unwanted elements of the image may be considered “4D noise.”
One goal of 4D processing is to attenuate 4D noise. In certain systems, 4D noise can be caused by changes in the environmental or seismic imaging equipment over time. For example, changes in temperature, changes in moisture, or shifting particles may change the velocity, amplitude, frequency characteristics, or other aspects of seismic wave propagation. As another example, certain properties of the receivers may change over time, which can affect the recorded signal. Since these changes can distort seismic images, 3D images from different surveys can show differences that result from environmental or equipment changes rather than the structural changes in the layers or reservoir that are relevant to production. 4D images are therefore rendered less accurate by the destabilizing effects of changes in the environment or equipment.